CN111837257B - Lithium secondary battery - Google Patents

Lithium secondary battery Download PDF

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Publication number
CN111837257B
CN111837257B CN201980018114.XA CN201980018114A CN111837257B CN 111837257 B CN111837257 B CN 111837257B CN 201980018114 A CN201980018114 A CN 201980018114A CN 111837257 B CN111837257 B CN 111837257B
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electrolyte
electrolyte layer
equal
lithium
secondary battery
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CN111837257A (en
Inventor
朴恩囧
张民哲
尹锡逸
孙炳国
朴昶勋
金度延
郑普逻
咸艺麟
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Lg Energy Solution
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LG Energy Solution Ltd
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Priority claimed from PCT/KR2019/014570 external-priority patent/WO2020091448A1/en
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    • H—ELECTRICITY
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
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Abstract

The present application relates to a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte includes a first electrolyte layer facing the negative electrode and a second electrolyte layer disposed on the first electrolyte layer and facing the positive electrode, the first electrolyte layer has a higher ionic conductivity than the second electrolyte layer, and lithium ions migrate out of the positive electrode by charging to form lithium metal on a negative electrode collector of the negative electrode.

Description

Lithium secondary battery
Technical Field
The present application claims the benefit of priority based on korean patent application No. 10-2018-013581 filed on day 31 of 10 in 2018 and korean patent application No. 10-2019-0137828 filed on day 31 of 10 in 2019, the entire disclosures of which are incorporated herein by reference.
The present application relates to a lithium secondary battery having a negative electrode-free structure, which includes electrolytes having different ion conductivities.
Background
Various devices requiring a battery from a portable phone, a wireless home appliance to an electric vehicle have been recently developed, and as these devices are developed, the demand for secondary batteries is also increasing. In particular, with the trend toward miniaturization of electronic products, secondary batteries also tend to be light-weighted and miniaturized.
In response to this trend, lithium secondary batteries using lithium metal as an active material have recently been attracting attention. Lithium metal has a low oxidation-reduction potential (3.045V relative to a standard hydrogen electrode) and a high gravimetric energy density (3,860 mAhg) -1 ) And has been expected as a negative electrode material for a high-capacity secondary battery.
However, when lithium metal is used as a negative electrode of a battery, a lithium foil is generally attached to a flat current collector to manufacture the battery, and since lithium has high reactivity as an alkali metal to explosively react with water and also react with oxygen in the atmosphere, there is a disadvantage in that it is difficult to manufacture and use in a general environment. In particular, when lithium metal is exposed to the atmosphere, lithium metal such as LiOH, li are obtained due to oxidation 2 O and Li 2 CO 3 And an iso-oxide layer. When a surface oxide layer (natural layer) is present on the surface, the oxide layer serves as an insulating film, thereby decreasing conductivity, and causing an increase in resistance by suppressing smooth migration of lithium ionsProblems.
For this reason, the problem of forming a surface oxide layer due to lithium metal reactivity has been partially improved by performing a vacuum evaporation process in forming a lithium anode, however, exposure to the atmosphere during battery assembly remains impossible to fundamentally inhibit the formation of the surface oxide layer. In view of the above, there is a need to develop a lithium metal electrode capable of solving the lithium reactivity problem and simplifying the process while improving energy efficiency by using lithium metal.
Prior art literature
Patent literature
Korean patent publication No. 10-2016-0052323 lithium electrode and lithium battery comprising the same "
Disclosure of Invention
Technical problem
As a result of extensive studies in view of the above problems, the inventors of the present application have devised a negative electrode-free battery structure capable of forming a lithium metal layer on a negative electrode current collector using lithium ions transferred from a positive electrode active material by charging after assembling a battery, so that contact of lithium metal with the atmosphere is fundamentally prevented at the time of assembling the battery, and have developed a composition of a positive electrode active material capable of stably forming a lithium metal layer. In addition, the inventors of the present application have developed a lithium secondary battery capable of suppressing dendrite growth by a difference in ion conductivity by including two or more electrolyte layers having different ion conductivities.
Accordingly, an object of the present application is to provide a lithium secondary battery that improves performance and life by solving problems caused by lithium metal reactivity and problems generated during assembly.
Technical proposal
In order to achieve the above object, the present application provides a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte,
wherein the electrolyte includes a first electrolyte layer facing the negative electrode and a second electrolyte layer disposed on the first electrolyte layer and facing the positive electrode,
the first electrolyte layer has a higher ionic conductivity than the second electrolyte layer, and
by charging, lithium ions migrate out of the positive electrode to form lithium metal on the negative electrode current collector of the negative electrode.
Advantageous effects
The lithium secondary battery of the present application is coated while being isolated from the atmosphere by the process of forming the lithium metal layer on the negative electrode current collector, and therefore can suppress the formation of a surface oxide layer on the lithium metal due to oxygen and moisture in the atmosphere, and as a result, an effect of improving cycle life characteristics is obtained.
In addition, by including two or more electrolyte layers having different ionic conductivities, an effect of significantly suppressing dendrite growth by the difference in ionic conductivities is obtained.
Drawings
Fig. 1 is a simulation diagram of a lithium secondary battery manufactured according to the present application.
Fig. 2 is a simulation diagram of a lithium secondary battery manufactured according to the present application after initial charge is completed.
Fig. 3 schematically shows the structure and mechanism of a conventional lithium secondary battery.
Fig. 4 schematically shows the structure and mechanism of the lithium secondary battery of the present application.
Detailed Description
Hereinafter, the present application will be described in more detail.
In the drawings, for clarity of description of the application, parts irrelevant to the description are not included, and like reference numerals are used for like elements throughout the specification. In addition, the dimensions and relative dimensions of the elements in the figures are not related to actual proportions and may be reduced or exaggerated for clarity of description.
The terms or words used in the present specification and claims should not be construed as limited to general or dictionary meanings, but should be construed as meanings and concepts corresponding to technical ideas of the present disclosure based on the principle that an inventor can appropriately define the concepts of the terms so as to describe the present application in the best possible manner.
In this specification, a layer mentioned as being "on" another layer or substrate may be formed directly on the other layer or substrate, or with a third layer provided therebetween. In addition, in this specification, directional expressions such as upward, upward (portion), or upper surface may be understood as meaning downward, downward (portion), or lower surface, etc., based on standards. In other words, the expression spatial direction needs to be understood as relative direction and need not be interpreted restrictively as representing absolute direction.
In addition, terms such as "comprising," "including," or "having" are intended to specify the presence of features, numbers, components, or combinations thereof described in the specification, and are to be construed as not excluding the possibility of the presence or addition of one or more other features, numbers, components, or combinations thereof.
In the drawings, the thickness of layers and regions may be exaggerated or omitted for clarity, and like reference numerals denote like components throughout the specification.
In addition, when the present application is described hereinafter, a detailed description will not be given when it is considered that a detailed description of related known functions or constructions unnecessarily obscure the gist of the present application.
The present application relates to a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte,
wherein the electrolyte includes a first electrolyte layer facing the negative electrode and a second electrolyte layer disposed on the first electrolyte layer and facing the positive electrode,
the first electrolyte layer has a higher ionic conductivity than the second electrolyte layer, and
by charging, lithium ions migrate out of the positive electrode to form lithium metal on the negative electrode current collector of the negative electrode.
Fig. 1 is a sectional view of a lithium secondary battery manufactured according to a first embodiment of the present application, and is provided with a positive electrode including a positive electrode current collector 11 and a positive electrode mixture 13; a negative electrode current collector 21; a first electrolyte layer 31 facing the anode; and a second electrolyte layer 33 disposed on the first electrolyte layer and facing the positive electrode.
In the negative electrode of the common lithium secondary battery, the negative electrode is formed on the negative electrode collector 21, however, in the present application, the negative electrode-free battery structure is assembled using only the negative electrode collector 21, the first electrolyte layer 31, and the second electrolyte layer 33, and then a lithium metal layer (not shown) is formed as a negative electrode mixture between the negative electrode collector 21 and the first electrolyte layer 31 by charging lithium ions deintercalated from the positive electrode mixture 13, as a result, a negative electrode having a known negative electrode collector/negative electrode mixture configuration is formed, thereby obtaining the configuration of the common lithium secondary battery.
More specifically, a lithium metal layer may be formed in the first electrolyte layer 31 formed on the anode current collector 21.
In other words, in the present application, the lithium secondary battery is a concept including both a battery without a negative electrode formed on a negative electrode collector at the time of initial assembly of the battery, or a battery in which a negative electrode is formed on a negative electrode collector so as to have a negative electrode according to use.
In addition, in the anode of the present application, the form of lithium metal formed as an anode mixture on the anode current collector includes a porous structure in which lithium metal is formed in a layer and lithium metal is not formed in a layer (for example, a structure in which lithium metal is aggregated in a particle form).
Hereinafter, the present application will be described in terms of the lithium metal layer 23 formed in layers based on lithium metal, however, it is apparent that such description does not exclude a structure in which lithium metal is not formed in layers.
Fig. 2 is a simulation diagram of a lithium secondary battery manufactured according to the first embodiment of the present application after initial charge is completed.
According to fig. 2, when a voltage of a certain level or more is applied to a lithium secondary battery having a non-negative electrode battery structure to charge, lithium ions are released from the positive electrode mixture 13 of the positive electrode 10, and these ions migrate to the negative electrode current collector 21 side after passing through the second electrolyte layer 33 and the first electrolyte layer 31, and a lithium metal layer 23 formed of lithium alone is formed on the negative electrode current collector 21, thereby forming the negative electrode 20.
Such a lithium metal layer 23 formed by charging has an advantage of very easy adjustment of interface characteristics when compared with a conventional anode obtained by sputtering the lithium metal layer 23 on the anode current collector 21 or laminating a lithium foil and the anode current collector 21.
In particular, by being formed in a negative electrode-free battery structure, lithium metal is not exposed to the atmosphere at all during battery assembly, which fundamentally prevents existing problems such as formation of an oxide layer on the surface due to high reactivity of lithium itself and thus causes a reduction in life of the lithium secondary battery.
In the negative electrode-less battery structure of the present application, the negative electrode current collector 21 forming the negative electrode is generally made to a thickness of 3 μm to 50 μm.
The negative electrode current collector 21 capable of forming the lithium metal layer 23 by charging is not particularly limited as long as it has conductivity without causing chemical changes in the lithium secondary battery. As examples, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, or the like can be used.
At this time, the negative electrode current collector 21 may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric, on the surface of which microscopic irregularities are formed.
The electrolyte of the lithium secondary battery of the present application includes a first electrolyte layer and a second electrolyte layer. The first electrolyte layer faces the negative electrode, the second electrolyte layer is disposed on the first electrolyte layer and the second electrolyte layer faces the positive electrode.
In addition, the first electrolyte layer has a higher ionic conductivity than the second electrolyte layer.
In the electrolyte of the present application, the ionic conductivity of the first electrolyte layer may be 10 -5 S/cm to 10 -2 S/cm, the ionic conductivity of the second electrolyte layer may be 10 -6 S/cm to 10 -3 S/cm。
In addition, the difference in ion conductivity between the first electrolyte layer and the second electrolyte layer may be 2 to 10 times 4 And more preferably 10 to 100 times.
When the difference in ion conductivity is less than the lower limit of the above range, the effect of suppressing the growth of lithium dendrites becomes insignificant, and a difference greater than the upper limit is not preferable because the battery driving efficiency is lowered.
In addition, the first electrolyte layer of the present application has lower strength than the second electrolyte layer. The strength of the first electrolyte layer may be 10 5 Pa or less, the strength of the second electrolyte layer may be greater than 10 5 Pa. Preferably, the strength of the first electrolyte layer may be 10Pa to 10Pa 4 Pa, the strength of the second electrolyte layer may be 10 6 Pa to 10 10 Pa。
Lithium secondary batteries using lithium metal or a material containing lithium metal as a negative electrode for lithium secondary batteries generally deteriorate rapidly first due to lithium dendrite growth, reactivity of lithium with an electrolyte, and other side reactions. Second, when a protective layer is formed on the surface of the anode to solve the above-described problems and defects are generated in the protective layer, lithium dendrite growth is accelerated in the defect generating part, thereby causing a short circuit of the battery.
As a result of extensive studies to solve these problems, the inventors of the present application have found that, when a first electrolyte layer has higher ion conductivity than a second electrolyte layer, lithium ions are not concentrated at defective sites even when defects are generated in the first electrolyte layer (fig. 3), and lithium is plated through the first electrolyte layer having higher ion conductivity around the defect generating portion (fig. 4), thereby preventing rapid growth of lithium dendrites, and completed the present application.
Therefore, in the electrolyte of the lithium secondary battery of the present application, the first electrolyte layer (or protective layer) facing the negative electrode has higher ion conductivity than the second electrolyte layer.
The protective layer needs to satisfy the condition that lithium ions can migrate while no current flows, and thus can be understood as an electrolyte layer. Thus, in the present application, the first electrolyte layer is defined to also have the function of a protective layer.
In the electrolyte of the present application, one or more of the first electrolyte layer and the second electrolyte layer is a semi-solid electrolyte or a solid electrolyte. This is due to the fact that: when both the first electrolyte layer and the second electrolyte layer are liquid, they may be mixed, making it difficult to obtain a target effect in the present application.
In the electrolyte of the present application, the thickness of the first electrolyte layer may be 0.1 μm to 20 μm, preferably 0.1 μm to 10 μm. When the thickness is less than 0.1 μm, the function as a protective layer may not be achieved, and when the thickness is more than 20 μm, the interfacial resistance increases, resulting in degradation of battery characteristics.
In addition, the thickness of the second electrolyte layer may be 0.1 μm to 50 μm, preferably 0.1 μm to 30 μm. When the thickness is less than 0.1 μm, the function as an electrolyte may not be achieved, and when the thickness is more than 50 μm, the interfacial resistance increases, resulting in degradation of battery characteristics.
In the electrolyte of the present application, the electrolyte may further include one or more electrolyte layers formed on the second electrolyte layer. In this case, the ionic conductivity of one or more electrolyte layers may be higher than that of the first electrolyte layer, and when the ionic conductivity is higher, the driving performance of the battery may be further improved. This is because the object of the present application can be achieved by the ion conductivity relationship between the first electrolyte layer and the second electrolyte layer.
In the electrolyte of the present application, among the one or more electrolyte layers formed on the second electrolyte layer, the electrolyte layer facing the positive electrode may have higher ion conductivity than the second electrolyte layer.
When the electrolyte layer facing the positive electrode has high ion conductivity, li ions are rapidly inserted into and released from the positive electrode, so that the resistance decreases during charge and discharge, and as a result, the driving performance of the battery, for example, the rate characteristics of the battery, can be further improved.
In the electrolyte of the present application, among the one or more electrolyte layers formed on the second electrolyte layer, the ion conductivity of the electrolyte layer facing the positive electrode may be 10 -5 S/cm to 10 -2 S/cm, more preferably 10 -4 S/cm to 10 -2 S/cm。
In one embodiment of the present application, one or more electrolyte layers formed on the second electrolyte layer are formed of one electrolyte layer, and the electrolyte layer may have a form facing the positive electrode.
The electrolyte of the present application may be provided with a separator between the electrolytes. In addition, in this case, the separator may be provided in a form immersed in the electrolyte.
In one embodiment of the present application, a separator may be formed on the second electrolyte layer. However, the structure is not limited to this form.
In the electrolyte of the present application, the first electrolyte layer is preferably formed of a semi-solid electrolyte or a solid electrolyte in view of the function as a protective layer. As the semi-solid electrolyte and the solid electrolyte, those known in the art may be used without limitation as long as they meet the ionic conductivity conditions defined above.
In the electrolyte of the present application, the second electrolyte layer may be formed as a liquid electrolyte, a semi-solid electrolyte, or a solid electrolyte. As the liquid electrolyte, the semi-solid electrolyte, and the solid electrolyte, electrolytes known in the art may be used without limitation as long as they meet the ionic conductivity conditions defined above.
The liquid electrolyte, the semi-solid electrolyte, and the solid electrolyte may have the following forms, for example, however, the forms are not limited thereto.
The nonaqueous electrolyte containing a lithium salt is formed of a lithium salt and an electrolyte, and as the electrolyte, a nonaqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, and the like are used.
The lithium salt of the present application may be, for example, a material selected from the group consisting of LiNO 3 、LiSCN、LiCl、LiBr、LiI、LiPF 6 、LiBF 4 、LiSbF 6 、LiAsF 6 、LiB 10 Cl 10 、LiCH 3 SO 3 、LiCF 3 SO 3 、LiCF 3 CO 2 、LiClO 4 、LiAlCl 4 、Li(Ph) 4 、LiC(CF 3 SO 2 ) 3 、LiN(FSO 2 ) 2 、LiN(CF 3 SO 2 ) 2 、LiN(C 2 F 5 SO 2 ) 2 、LiN(SFO 2 ) 2 、LiN(CF 3 CF 2 SO 2 ) 2 More than one of the group consisting of lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenyl borate, lithium imide, and combinations thereof.
The concentration of lithium salt may be 0.2M to 3M, specifically 0.6M to 2M, more specifically 0.7M to 1.7M, depending on various factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charge and discharge conditions of the battery, the operating temperature, and other factors known in the lithium battery art. When the amount is less than 0.2M, the conductivity of the electrolyte may be lowered, resulting in a decrease in the electrolyte performance, and when the amount is more than 3M, the viscosity of the electrolyte is increased, resulting in lithium ions (Li + ) Mobility decreases.
Nonaqueous organic solvents need to dissolve lithium salts well, and examples of such nonaqueous organic solvents may include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylene carbonate, γ -butyrolactone, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, tetrahydroxyflange g (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1, 3-dioxolane, 4-methyl-1, 3-dioxane, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphotriester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1, 3-dimethyl-2-imidazolidone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, or ethyl propionate, and the organic solvents may be used alone or as a mixture of two or more organic solvents.
As the organic solid electrolyte, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, polylysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ion dissociating groups, and the like can be used.
As the inorganic solid electrolyte, for example, a nitride, a halide, a sulfate, or the like of Li can be usedFor example Li 3 N、LiI、Li 5 NI 2 、Li 3 N-LiI-LiOH、LiSiO 4 、LiSiO 4 -LiI-LiOH、Li 2 SiS 3 、Li 4 SiO 4 、Li 4 SiO 4 -LiI-LiOH or Li 3 PO 4 -Li 2 S-SiS 2 。
In order to improve charge and discharge characteristics and flame retardancy, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, N-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, or the like may be further added to the electrolyte of the present application. In some cases, in order to provide incombustibility, halogen-containing solvents such as carbon tetrachloride and trifluoroethylene may be further included, carbon dioxide gas may be further included in order to improve high-temperature storage characteristics, and fluoroethylene carbonate (FEC), propylene sultone (PRS), fluoropropylene carbonate (FPC), and the like may be further included.
Meanwhile, as for the positive electrode mixture 13, various positive electrode active materials may be used depending on the type of battery, and the positive electrode active material used in the present application is not particularly limited as long as it is a material capable of inserting or extracting lithium ions, however, lithium transition metal oxide is currently generally used as a positive electrode active material capable of obtaining a battery excellent in life characteristics and charge and discharge efficiency.
As the lithium transition metal oxide, for example, lithium cobalt oxide (LiCoO) containing two or more transition metals and substituted with one or more transition metals, for example, may be included 2 ) Or lithium nickel oxide (LiNiO) 2 ) Layered compounds of the like; lithium manganese oxide substituted with one or more transition metals; lithium nickel-based oxide; spinel-based lithium nickel manganese composite oxides; spinel-based lithium manganese oxide in which a part of Li of the chemical formula is substituted with an alkaline earth metal; and olivine-based lithium metal phosphates, etc., however, the lithium transition metal oxide is not limited thereto.
Preferably, lithium transition metal oxygen is usedCompounds selected from LiCoO, for example, can be used 2 、LiNiO 2 、LiMnO 2 、LiMn 2 O 4 、Li 2 NiO 2 、Li(Ni a Co b Mn c )O 2 (0＜a＜1，0＜b＜1，0＜c＜1，a+b+c＝1)、LiNi 1-Y Co Y O 2 、LiCo 1-Y Mn Y O 2 、LiNi 1-Y Mn Y O 2 (wherein, Y is more than or equal to 0 and less than 1), li (Ni) a Co b Mn c )O 4 (0＜a＜2，0＜b＜2，0＜c＜2，a+b+c＝2)、LiMn 2-z Ni z O 4 、LiMn 2-z Co z O 4 (wherein 0 < Z < 2), li x M y Mn 2-y O 4-z A z (wherein, x is more than or equal to 0.9 and less than or equal to 1.2, 0)<y<2，0≤z<0.2, M= Al, mg, ni, co, fe, cr, V, ti, cu, B, ca, zn, zr, nb, mo, sr, sb, W, ti and Bi, A is one or more anions of-1 or-2 valence), li 1+a Ni b M’ 1-b O 2-c A’ c (wherein a is more than or equal to 0 and less than or equal to 0.1, b is more than or equal to 0 and less than or equal to 0.8, and c is more than or equal to 0)<0.2, M 'is one or more types selected from the group consisting of octahedral stabilization elements such as Mn, co, mg or Al, A' is one or more anions of-1 or-2 valency), liCoPO 4 And LiFePO 4 One or more types of the group consisting of LiCoO is most preferably used 2 . In addition to these oxides, sulfides, selenides, halides, and the like can be used.
Lithium transition metal oxide is used in the positive electrode mixture 13 as a positive electrode active material together with a binder, a conductor, and the like. In the non-negative electrode battery structure of the present application, the lithium source used to form the lithium metal layer 23 is a lithium transition metal oxide. In other words, when charging is performed in a specific voltage range, lithium ions in the lithium transition metal oxide are released, thereby forming the lithium metal layer 23 on the negative electrode current collector 21.
However, since lithium ions in the lithium transition metal oxide are not easily released themselves or lithium which is not charged and discharged does not exist at the above-described operating voltage level, it is very difficult to form the lithium metal layer 23, and when only the lithium transition metal oxide is used, the irreversible capacity is greatly reduced, thereby causing a problem of a decrease in capacity and life characteristics of the lithium secondary battery.
In view of the above, in the present application, when primary charging is performed at 0.01C to 0.2C in a voltage range of 4.5V to 2.5V, a lithium metal compound as a highly irreversible material having an initial charge capacity of 200mAh/g or more or an initial irreversible degree of 30% or more is used together as an additive capable of providing a lithium source to a lithium transition metal oxide.
The term "highly irreversible material" mentioned in the present application may be used in the same manner as the "high-capacity irreversible material" in another term, and this means a material having a high irreversible capacity ratio at the first charge-discharge cycle, i.e., (first-cycle charge capacity-first-cycle discharge capacity)/first-cycle charge capacity ". In other words, a highly irreversible material can irreversibly provide excess lithium ions during the first charge-discharge cycle. For example, among lithium transition metal compounds capable of inserting and extracting lithium ions, a positive electrode material having a large irreversible capacity (first-cycle charge capacity—first-cycle discharge capacity) in the first charge-discharge cycle can be considered.
The irreversible capacity of the positive electrode active material generally used is about 2% to 10% with respect to the initial charge capacity, but in the present application, a lithium metal compound as a highly irreversible material, that is, a lithium metal compound having an initial degree of irreversible of 30% or more, preferably 50% or more of the initial charge capacity, may be used together. Further, as the lithium metal compound, a material having an initial charge capacity of 200mAh/g or more, preferably 230mAh/g or more can be used. The use of such a lithium metal compound serves as a lithium source capable of forming the lithium metal layer 23 while improving the irreversible capacity of the lithium transition metal oxide as a positive electrode active material.
As the lithium metal compound provided in the present application, compounds represented by the following chemical formulas 1 to 8 may be used.
[ chemical formula 1]
Li 2 Ni 1-a M 1 a O 2
(wherein a is 0.ltoreq.a < 1, M 1 One or more types of elements selected from the group consisting of Mn, fe, co, cu, zn, mg and Cd),
[ chemical formula 2]
Li 2+b Ni 1-c M 2 c O 2+d
(in the formula, b is more than or equal to-0.5 and less than or equal to-0.5, c is more than or equal to 0 and less than or equal to 1, d is more than or equal to 0 and less than 0.3, M 2 One or more types of elements selected from the group consisting of P, B, C, al, sc, sr, ti, V, zr, mn, fe, co, cu, zn, cr, mg, nb, mo and Cd),
[ chemical formula 3]
LiM 3 e Mn 1-e O 2
(wherein e is 0.ltoreq.e < 0.5, M) 3 One or more types of elements selected from the group consisting of Cr, al, ni, mn and Co),
[ chemical formula 4]
Li 2 M 4 O 2
(wherein M 4 One or more types of elements selected from the group consisting of Cu and Ni),
[ chemical formula 5]
Li 3+f Nb 1-g M 5 g S 4-h
(in the formula, f is more than or equal to-0.1 and less than or equal to-1, g is more than or equal to-0.5, h is more than or equal to-0.1 and less than or equal to-0.5, M 5 One or more types of elements selected from the group consisting of Mn, fe, co, cu, zn, mg and Cd),
[ chemical formula 6]
LiM 6 i Mn 1-i O 2
(wherein i is 0.05.ltoreq.i < 0.5, M) 6 One or more types of elements selected from the group consisting of Cr, al, ni, mn and Co),
[ chemical formula 7]
LiM 7 2j Mn 2-2j O 4
(wherein j is 0.05.ltoreq.j)＜0.5，M 7 One or more types of elements selected from the group consisting of Cr, al, ni, mn and Co), and
[ chemical formula 8]
Li k M 8 m N n
(wherein M 8 Represents an alkaline earth metal, k/(k+m+n) is 0.10 to 0.40, m/(k+m+n) is 0.20 to 0.50, and n/(k+m+n) is 0.20 to 0.50.
The lithium metal compounds of chemical formulas 1 to 8 differ in irreversible capacity according to structures. They may be used alone or as a mixture, and function to increase the irreversible capacity of the positive electrode active material.
As one example, the highly irreversible materials represented by chemical formula 1 and chemical formula 3 have different irreversible capacities according to types, and as one example, have values listed in table 1 below.
TABLE 1
In addition, the lithium metal compound of chemical formula 2 preferably belongs to the space group Immm, and in this group, more preferably, the Ni and M composite oxide forms a planar quadrangle (Ni, M) O 4 Wherein the planar quadrilateral structures form a backbone while sharing facing edges (edges formed by O-O). The lattice constant of the compound of chemical formula 2 is preferably α=90 °, β=90° and γ=90°.
In addition, in the lithium metal compound of chemical formula 8, the content of the alkaline earth metal is 30 to 45 at%, and the content of nitrogen is 30 to 45 at%. At this time, when the alkaline earth metal content and the nitrogen content are within the above ranges, the compound of chemical formula 8 has excellent thermal characteristics and lithium ion conductive characteristics. In chemical formula 8, k/(k+m+n) is 0.15 to 0.35, for example, 0.2 to 0.33, m/(k+m+n) is 0.30 to 0.45, for example, 0.31 to 0.33, and n/(k+m+n) is 0.30 to 0.45, for example, 0.31 to 0.33.
According to one embodiment, in the electrode active material of chemical formula 1, a is 0.5 to 1, b is 1, and c is 1.
The surface of the positive electrode active material may have a core-shell structure coated with a compound of any one of chemical formulas 1 to 8.
When a coating layer formed of the compound of any one of chemical formulas 1 to 8 is formed on the surface of the core active material, the electrode active material exhibits stable characteristics even in an environment where lithium ions are continuously inserted and released, while maintaining low resistance characteristics.
In the electrode active material of the embodiment of the present application, the thickness of the coating layer is 1nm to 100nm. When the thickness of the coating layer is within the above range, the ion-conductive property of the electrode active material is excellent.
The average particle diameter of the electrode active material is 1 μm to 30 μm, and according to one embodiment, 8 μm to 12 μm. When the average particle diameter of the positive electrode active material is within the above range, the battery capacity characteristics are excellent.
Examples of alkaline earth metal doped core active materials may include magnesium doped LiCoO 2 . The magnesium content is 0.01 to 3 parts by weight based on 100 parts by weight of the core active material.
Lithium transition metal oxide is used in the positive electrode mixture 13 as a positive electrode active material together with a binder, a conductor, and the like. In the structure of the negative electrode-less battery of the present application, the lithium source for forming the lithium metal layer 23 is a lithium transition metal oxide. In other words, when charging is performed in a specific voltage range, lithium ions in the lithium transition metal oxide are released, thereby forming the lithium metal layer 23 on the negative electrode current collector 21.
In the present application, for the charging range for forming the lithium metal layer 23, one charge is performed at 0.01C to 0.2C in the voltage range of 4.5V to 2.5V. When the charge is performed at a voltage level less than the above range, it is difficult to form the lithium metal layer 23, whereas when the voltage level is greater than the above range, the battery (cell) is damaged, and the charge and discharge cannot be performed normally after the overdischarge occurs.
The lithium metal layer 23 formed above forms a uniform continuous or discontinuous layer on the anode current collector 21. As one example, when the anode current collector 21 has a foil form, a continuous thin film form may be obtained, and when the anode current collector 21 has a three-dimensional porous structure, the lithium metal layer 23 may be discontinuously formed. In other words, the discontinuous layer refers to a form of discontinuous distribution in which regions having the lithium metal layer 23 and regions not having the lithium metal layer 23 exist in a specific region, and the regions having the lithium metal layer 23 are discontinuous in distribution by dividing, breaking, or separating the regions having the lithium compound into islands.
The lithium metal layer 23 formed by such charge and discharge has a thickness of at least 50nm or more and 100 μm or less, preferably 1 μm to 50 μm to serve as a negative electrode. When the thickness is less than the above range, the battery charge and discharge efficiency is rapidly lowered. In contrast, when the thickness is larger than the above range, life characteristics and the like are stabilized, however, there is a problem in that the battery energy density is lowered.
In particular, a non-negative electrode battery having no lithium metal is manufactured at the time of assembling the battery, so that an oxide layer due to high reactivity of lithium generated during the assembling process is not or hardly formed on the lithium metal layer 23 provided in the present application, compared to the conventional lithium secondary battery assembled using a lithium foil. As a result, deterioration in the life of the battery caused by the oxide layer can be prevented.
In addition, the lithium metal layer 23 is moved by the charge of the highly irreversible material, and this can form a more stable lithium metal layer 23 than the formation of the lithium metal layer 23 on the positive electrode. When lithium metal is attached to the positive electrode, chemical reaction may occur between the positive electrode and the lithium metal.
The positive electrode mixture 13 is formed by containing a positive electrode active material and a lithium metal compound, and in this case, the positive electrode mixture 13 may further include a conductor, a binder, and other additives commonly used in lithium secondary batteries.
The conductor is used to further improve the conductivity of the electrode active material. Such a conductor is not particularly limited as long as it has conductivity without causing chemical changes of the corresponding battery, and for example, graphite such as natural graphite or artificial graphite may be used; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers, such as carbon fibers or metal fibers; a fluorocarbon compound; metal powders, such as aluminum and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; polyphenylene derivatives, and the like.
A binder may be further included for binding the positive electrode active material, the lithium metal compound, and the conductor, and binding to the current collector. The binder may include a thermoplastic resin or a thermosetting resin. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, vinylidene fluoride-pentafluoropropene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, and ethylene-acrylic acid copolymer, etc. may be used alone or as a mixture, however, the adhesive is not limited thereto, and those which can be used as an adhesive in the art may be used.
Examples of other additives may include fillers. The filler is selectively used as a component for inhibiting the expansion of the electrode, and is not particularly limited as long as it is a fibrous material without causing chemical changes of the corresponding battery. For example, an olefin-based polymer such as polyethylene or polypropylene, or a fibrous material such as glass fiber or carbon fiber may be used.
The positive electrode mixture 13 of the present application is formed on the positive electrode current collector 11.
The positive electrode current collector is generally made to a thickness of 3 μm to 500 μm. Such a positive electrode collector 11 is not particularly limited as long as it has high conductivity without causing chemical changes in the lithium secondary battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like may be used.
In this case, in order to improve the adhesive strength with the positive electrode active material, the positive electrode current collector 11 may be used in various forms, for example, a film, sheet, foil, net, porous body, foam, or nonwoven fabric having microscopic irregularities formed on the surface thereof.
The method of applying the positive electrode mixture 13 to the current collector may include a method of distributing the electrode mixture slurry on the current collector and uniformly dispersing the resultant using a doctor blade or the like, a die casting method, a comma coating method, a screen printing method, and the like. In addition, after being molded on a separate substrate, the electrode mixture slurry may be bonded to the current collector by using a pressing or lamination method, however, the method is not limited thereto.
The separator used in the lithium secondary battery of the present application separates or insulates the positive electrode and the negative electrode from each other and enables lithium ions to be transferred between the positive electrode and the negative electrode, and may be formed of a porous non-conductive or insulating material. As an insulator having high ion permeability and mechanical strength, such a separator may be a separate member such as a thin film or a membrane, or a coating added to the positive electrode and/or the negative electrode. In addition, when a solid electrolyte such as a polymer or the like is used as the electrolyte, the solid electrolyte may also be used as the separator.
The separator preferably has a pore diameter of usually 0.01 μm to 10 μm and a thickness of usually 5 μm to 300 μm, and as such a separator, a glass electrolyte, a polymer electrolyte, a ceramic electrolyte, or the like can be used. For example, olefin-based polymers such as polypropylene having chemical resistance and hydrophobicity, sheets made of glass fiber or polyethylene, nonwoven fabrics, kraft paper, and the like are used. Typical examples of commercially available products may include the Celgard series (Celgard R 2400. 2300,Hoechst Celanese Corp), polypropylene separators (product of Ube Industries ltd. Or product of Pall RAI), polyethylene series (ton or Entek), and the like.
The electrolyte separator in a solid state may include about less than 20% by weight of a non-aqueous organic solvent, in which case a suitable gelling agent may be further included to reduce the fluidity of the organic solvent. Typical examples of such gelling agents may include polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, and the like.
The manufacture of the lithium secondary battery having the above-described configuration is not particularly limited in the present application, and the lithium secondary battery may be manufactured using a known method.
As one example, in an all-solid battery type, the electrolyte of the present application is placed between a positive electrode and a negative electrode, and the result is compression molded to assemble the battery cell.
The assembled cell is mounted in an external material and then sealed by heat compression or the like. As the exterior material, a laminated bag of aluminum, stainless steel, or the like, or a cylindrical or prismatic metal-based container can be suitably used.
Hereinafter, preferred embodiments will be provided to illustrate the present application, however, the following embodiments are for illustrative purposes only, and it will be apparent to those skilled in the art that various changes and modifications may be made within the scope and technical spirit of the present application, and that such changes and modifications also fall within the scope of the appended claims.
< production of non-negative electrode lithium Secondary Battery >
Example 1
LCO (LiCoO) mixed in a weight ratio of 9:1 in N-methyl-2-pyrrolidone was used 2 ) And L 2 N(Li 2 NiO 2 ) As a positive electrode active material, and the positive electrode active material: conductor (super-P): after the binder (PVdF) was mixed in a weight ratio of 95:2.5:2.5, the resultant was mixed for 30 minutes using a paste mixer (paste face mixter), thereby manufacturing a slurry composition.
The slurry composition manufactured above was coated on a current collector (Al foil, thickness 20 μm) and dried at 130 ℃ for 12 hours, thereby manufacturing a load of 3mAh/cm 2 Is a positive electrode of (a).
The first electrolyte was manufactured by mixing LiFSI (2.8M) with dimethyl carbonate (DMC), and injected to the copper current collector, i.e., the negative electrode side to serve as the first electrolyte layer.
Poly (ethylene glycol) methyl ether acrylate (PEGMEA), poly (ethylene glycol) diacrylate (PEGDA), succinonitrile (SN) and LiTFSI were mixed in a weight ratio of 15:5:40:40 to produce a second electrolyte, and impregnated into a separator having a porosity of 48.8%, to form a second electrolyte layer. The second electrolyte layer is disposed between the first electrolyte layer and the third electrolyte layer.
By passing 1M LiPF 6 And 2 wt% of ethylene carbonate (VC) was dissolved in a mixed solvent in which Ethylene Carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:2:1 to manufacture a third electrolyte, and it was injected to the positive electrode side, thereby forming a third electrolyte layer.
The positive electrode manufactured above was placed on the third electrolyte layer, thereby manufacturing a negative electrode-free lithium secondary battery of example 1.
Example 2
A non-negative electrode lithium secondary battery of example 2 was fabricated in the same manner as in example 1, except that L was not used 2 N(Li 2 NiO 2 ) As the positive electrode active material.
Comparative example 1
LCO (LiCoO) mixed in a weight ratio of 9:1 in N-methyl-2-pyrrolidone was used 2 ) And L 2 N(Li 2 NiO 2 ) As a positive electrode active material, and the positive electrode active material: conductor (super-P): after mixing the binder (PVdF) at a weight ratio of 95:2.5:2.5, the resultant was mixed for 30 minutes using a paste mixer, thereby manufacturing a slurry composition.
The slurry composition manufactured above was coated on a current collector (Al foil, thickness 20 μm) and dried at 130 ℃ for 12 hours, thereby manufacturing a positive electrode.
By passing 1M LiPF 6 And 2 wt% of ethylene carbonate (VC) was dissolved in a mixed solvent in which Ethylene Carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:2:1 to manufacture an electrolyte, and it was injected into a separator having a porosity of 48.8% to be disposed at positive sideBetween the pole and the negative pole.
The positive electrode manufactured as described above was placed on a separator, thereby manufacturing a negative electrode-free lithium secondary battery of comparative example 1.
Experimental example 1 measurement of ion conductivity of electrolyte layer
The ion conductivities of each of example 1 and comparative example 1 were measured. The ionic conductivity of the first electrolyte layer and the ionic conductivity of the third electrolyte layer of example 1 were measured using a Mettler Toledo conductivity meter, and the ionic conductivity of the second electrolyte layer of example 1 was measured using a SUS/SUS cell. In addition, the ionic conductivity of the electrolyte layer of comparative example 1 was measured using a Mettler Toledo conductivity meter.
The results are shown in table 2 below.
TABLE 2
Experimental example 2 analysis of characteristics of lithium Secondary Battery
The non-negative lithium secondary batteries fabricated in example 1, example 2 and comparative example 1 were charged once at CC/CV (1C off current 5%) of 0.1C and 4.25V, respectively, to thereby fabricate lithium secondary batteries forming lithium metal layers.
Based on 3mAh/cm 2 Each lithium secondary battery was charged and discharged under conditions of 0.2C/0.5C, and the number of cycles when the capacity retention rate of the lithium secondary battery forming the lithium metal layer (23) was 50% or more with respect to the initial discharge capacity was measured. The results are shown in table 3 below.
TABLE 3
According to the results of Table 3, in the use of highly irreversible material L 2 In example 1 of N, no short circuit occurred, and the number of cycles at which the capacity retention rate was 50% or more relative to the initial discharge capacity was measured to be the highest, 17 cycles. In example 2, since highly irreversible material L is not used 2 N, the number of cycles thus measured was lower than in example 1. On the other hand, comparative example 1 contained only one electrolyte layer and a short circuit occurred at the 2 nd cycle, so that the capacity retention rate could not be measured and very unstable charge and discharge characteristics were obtained.
[ reference numerals ]
11: positive electrode current collector
13: positive electrode mixture
20: negative electrode
21: negative electrode current collector
23: lithium metal layer
31: a first electrolyte layer
33: a second electrolyte layer

Claims (12)

1. A lithium secondary battery, comprising:
a positive electrode;
a negative electrode; and
an electrolyte is provided, which is a metal-containing electrolyte,
wherein the electrolyte includes a first electrolyte layer facing the negative electrode and a second electrolyte layer disposed on the first electrolyte layer and facing the positive electrode;
the first electrolyte layer has a higher ionic conductivity than the second electrolyte layer; and is also provided with
Lithium ions migrate from the positive electrode to form lithium metal on a negative electrode current collector of the negative electrode by charging,
wherein the difference in ionic conductivity between the first electrolyte layer and the second electrolyte layer is 10-100 times,
wherein the positive electrode includes a lithium metal compound represented by any one of the following chemical formulas 1 to 8:
[ chemical formula 1]
Li 2 Ni 1-a M 1 a O 2
Wherein a is 0.ltoreq.a < 1, M 1 Is one or more types of elements selected from the group consisting of Mn, fe, co, cu, zn, mg and Cd,
[ chemical formula 2]
Li 2+b Ni 1-c M 2 c O 2+d
Wherein, -b is more than or equal to 0.5 and less than or equal to 0.5, c is more than or equal to 0 and less than or equal to 1, d is more than or equal to 0 and less than 0.3, M 2 Is one or more types of elements selected from the group consisting of P, B, C, al, sc, sr, ti, V, zr, mn, fe, co, cu, zn, cr, mg, nb, mo and Cd,
[ chemical formula 3]
LiM 3 e Mn 1-e O 2
Wherein e is more than or equal to 0 and less than 0.5, M 3 Is one or more types of elements selected from the group consisting of Cr, al, ni, mn and Co,
[ chemical formula 4]
Li 2 M 4 O 2
Wherein M is 4 Is one or more types of elements selected from the group consisting of Cu and Ni,
[ chemical formula 5]
Li 3+f Nb 1-g M 5 g S 4-h
In the formula, f is more than or equal to-0.1 and less than or equal to-1, g is more than or equal to-0.5, h is more than or equal to-0.1 and less than or equal to-0.5, M 5 Is one or more types of elements selected from the group consisting of Mn, fe, co, cu, zn, mg and Cd,
[ chemical formula 6]
LiM 6 i Mn 1-i O 2
Wherein i is 0.05-0.5, M 6 Is one or more types of elements selected from the group consisting of Cr, al, ni, mn and Co,
[ chemical formula 7]
LiM 7 2j Mn 2-2j O 4
Wherein j is 0.05.ltoreq.j < 0.5, M 7 Is one or more types of elements selected from the group consisting of Cr, al, ni, mn and Co, and
[ chemical formula 8]
Li k M 8 m N n
Wherein M is 8 Represents an alkaline earth metal, and k/(k+m+n) is 0.10 to 0.40, m/(k+m+n) is 0.20 to 0.50, and n/(k+m+n) is 0.20 to 0.50.
2. The lithium secondary battery according to claim 1, wherein the ionic conductivity of the first electrolyte layer is 10 -5 S/cm to 10 -2 S/cm, and
the ionic conductivity of the second electrolyte layer is 10 -6 S/cm to 10 -3 S/cm。
3. The lithium secondary battery according to claim 1, wherein one or more of the first electrolyte layer and the second electrolyte layer is a semi-solid electrolyte or a solid electrolyte.
4. The lithium secondary battery according to claim 1, wherein the first electrolyte layer has a thickness of 0.1 μm to 20 μm and the second electrolyte layer has a thickness of 0.1 μm to 50 μm.
5. The lithium secondary battery of claim 1, wherein the electrolyte further comprises one or more electrolyte layers formed on the second electrolyte layer.
6. The lithium secondary battery according to claim 5, wherein, among the one or more electrolyte layers formed on the second electrolyte layer, an electrolyte layer facing the positive electrode has higher ion conductivity than the second electrolyte layer.
7. The lithium secondary battery according to claim 6, wherein the one or more electrolyte layers formed on the second electrolyte layer are formed of one electrolyte layer.
8. The lithium secondary battery according to claim 6, wherein, among the one or more electrolyte layers formed on the second electrolyte layer, an electrolyte layer facing the positive electrode has an ion conductivity of 10 -5 S/cm to 10 -2 S/cm。
9. The lithium secondary battery according to claim 5, wherein the electrolyte has a separator disposed between the electrolytes.
10. The lithium secondary battery according to claim 9, wherein the separator is provided in a form impregnated into an electrolyte.
11. The lithium secondary battery according to claim 1, wherein the lithium metal is formed by one charge in a voltage range of 4.5V to 2.5V.
12. The lithium secondary battery according to claim 1, wherein the positive electrode comprises a material selected from the group consisting of LiCoO 2 、LiNiO 2 、LiMnO 2 、LiMn 2 O 4 、Li(Ni a Co b Mn c )O 2 Wherein 0 < a < 1,0 < b < 1,0 < c < 1, a+b+c=1, liNi 1-Y Co Y O 2 、LiCo 1-Y Mn Y O 2 、LiNi 1-Y Mn Y O 2 Wherein 0 < Y < 1, li (Ni) a Co b Mn c )O 4 Wherein 0 < a < 2,0 < b < 2,0 < c < 2, a+b+c=2, liMn 2-z Ni z O 4 、LiMn 2-z Co z O 4 Wherein Z is more than 0 and less than 2, li x M y Mn 2-y O 4-z A z Wherein x is more than or equal to 0.9 and less than or equal to 1.2,0<y<2，0≤z<0.2, M= Al, mg, ni, co, fe, cr, V, ti, cu, B, ca, zn, zr, nb, mo, sr, sb, W, ti and Bi, A is one or more anions of-1 or-2 valence, li 1+a Ni b M’ 1-b O 2-c A’ c Wherein a is more than or equal to 0 and less than or equal to 0.1, b is more than or equal to 0 and less than or equal to 0.8, and c is more than or equal to 0 and less than or equal to 0<0.2, M 'is one or more types selected from the group consisting of octahedral stabilization elements of Mn, co, mg or Al, A' is one or more anions of-1 or-2 valency, liCoPO 4 And LiFePO 4 One or more types of positive electrode active materials in the group consisting of.
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